The invention disclosed and claimed herein is directed to apparatus for supporting an X-ray tube anode for rotation. More particularly, the invention pertains to apparatus of such type having an element which is locked against rotational and radial movement, but is free to move axially to preload rotary bearings carrying the anode, and to take up expansion and contraction in the bearings resulting from heating effects.
The principal component of conventional X-ray equipment and computed tomography (CT) equipment is an X-ray tube which provides the source of X-rays. Such tubes contain a vacuum at 10.sup.-8 to 10.sup.-9 torr and operate by accelerating a stream of electrons from a heated cathode through a high voltage against a target anode. The conversion efficiencies of such tubes are low and therefore considerable heat is generated in the anode as a by-product of the X-ray generation.
In order to reduce heat concentration in the anode, the anode is mounted on an anode shaft and rotated at speeds up to 12,000 RPM, thereby continuously presenting the cathode a new and cooler surface. In a high performance X-ray tube, the surface of the anode may reach temperatures of 3,200.degree. C., and areas of the anode outside the immediate target surface may rise to temperatures of approximately 1,300.degree. C.
Much of the heat generated in the anode is radiated through the glass walls of the tube from high emissivity anode coatings. Even so, the anode shaft, as well as the support bearings which rotatably carry the shaft and anode, may rise to temperatures of up to 450.degree. C.
Each of the bearings for the anode shaft typically comprises a rolling contact ball bearing, that is, an annular train of rolling balls trapped between inner and outer races. The bearings are generally preloaded, to prolong bearing life. In a common arrangement, a front bearing is held fixed with respect to a stationary support member known as the anode stem, and the outer race of the rear bearing is carried on a cylindrical element known as a bearing retainer. The bearing retainer and rear bearing outer race are free to slide axially within a bore formed in the anode stem, and the rear bearings' inner race is fixed to the anode shaft. A preload spring applies an axial force to the rear bearing retainer to urge the outer bearing race rearwardly and to thereby apply a preloading force to the rear bearing. The axial force provided by the spring is also transmitted through the anode shaft to preload the front bearings. The preloading force improves the tracking of the bearing balls in their annular path between the inner and outer races of both front and rear bearings, increasing bearing life and reducing bearing noise. More particularly, the preloading provides a constant axial force (thrust load) on the bearings to distribute translated radial forces of the rotating anode between multiple rolling annular contact elements, that is, the balls rolling between the inner and outer races.
Arrangements of the above type are shown, for example, in commonly assigned U.S. Pat. No. 4,914,684, issued Apr. 3, 1990 to Kamleshwar Upadhya.
The axially slidable bearing retainer provides a further benefit in allowing axial expansion and contraction of the bearings, which occurs as the mechanism heats or cools. However, in order for the bearing retainer to move axially, it must be "slip fitted" within the bore of the anode stem, i.e., a slight clearance must be allowed between the bore wall and the outer cylindrical surface of the bearing retainer. Such clearance may be on the order of 0.001 inch to 0.003 inch by design, and stem bore out-of-roundness caused by machining can account for an additional 0.001 inch-0.002 inch in radial clearance. The resulting total clearance may be large enough to allow radial movement or "radial play" of the bearing retainer within the bore. Also, a rotational moment caused by stick/slip friction of the bearing can cause impacts of occur between the bearing retainer and an "anti-rotation" screw which, as shown in the Upadhya patent referred to above, is positioned between the bearing retainer and the anode stem to prevent the bearing retainer from rotating within the stem. Such undesirable mechanical movements are the primary cause of bearing noise in certain commercially important types of X-ray tubes. Also, vibration resulting therefrom can accelerate wear of contacting surfaces, and small metallic particles, caused by the wear, can become distributed in the bearings and even enter the vacuum tube itself. Such metallic particles have the effect of further increasing bearing noise, decreasing the life of the X-ray tube rotor and reducing high voltage stability for X-ray tube operation. Radial vibration between the rear bearing retainer and the stem can also create a high voltage current path from the anode stem to the bearing shaft, which would be highly undesirable.